Proton-conducting polymer membrane that contains polyazoles and is coated with a catalyst layer, and application thereof in fuel cells

ABSTRACT

The present invention relates to a proton-conducting polymer membrane which comprises polyazoles and is coated with a catalyst layer and is obtainable by a process comprising the steps A) preparation of a mixture comprising polyphosphoric acid, at least one polyazole (polymer A) and/or one or more compounds which are suitable for forming polyazoles under the action of heat according to step B), B) heating of the mixture obtainable according to step A) under inert gas to temperatures of up to 400° C., C) application of a layer using the mixture obtained according to step A) and/or B) to a support, D) treatment of the membrane formed in step C) until it is self-supporting, E) application of at least one catalyst-containing coating to the membrane formed in step C) and/or in step D).

The present invention relates to a proton-conducting polymer electrolytemembrane which comprises polyazole blends and is coated with a catalystlayer and can, owing to its excellent chemical and thermal properties,be used for a variety of purposes, in particular as polymer electrolytemembrane (PEM) in PEM fuel cells.

A fuel cell usually comprises an electrolyte and two electrodesseparated by the electrolyte. In the case of a fuel cell, a fuel such ashydrogen gas or a methanol/water mixture is supplied to one of the twoelectrodes and an oxidant such as oxygen gas or air is supplied to theother electrode and chemical energy from the oxidation of the fuel is inthis way converted directly into electric energy. The oxidation reactionforms protons and electrons.

The electrolyte is permeable to hydrogen ions, i.e. protons, but not toreactive fuels such as the hydrogen gas or methanol and the oxygen gas.

A fuel cell generally comprises a plurality of single cells known asMEUs (membrane-electrode unit) which each comprise an electrolyte andtwo electrodes separated by the electrolytes.

Electrolytes employed for the fuel cell are solids such as polymerelectrolyte membranes or liquids such as phosphoric acid. Recently,polymer electrolyte membranes have attracted attention as electrolytesfor fuel cells.

For example, polymer electrolyte membranes comprising complexes of basicpolymers and strong acids have been developed. Thus, WO96/13872 and thecorresponding U.S. Pat. No. 5,525,436 describe a process for producing aproton-conducting polymer electrolyte membrane, in which a basic polymersuch as polybenzimidazole is treated with a strong acid such asphosphoric acid, sulfuric acid, etc.

In J. Electrochem. Soc., Volume 142, No. 7, 1995, pp. L121-L123,describes doping of a polybenzimidazole in phosphoric acid.

In the case of the basic polymer membranes known from the prior art, themineral acid (usually concentrated phosphoric acid) used for achievingthe necessary proton conductivity is usually added after shaping of thepolyazole film. This polymer here serves as support for the electrolyteconsisting of the highly concentrated phosphoric acid. The polymermembrane in this case fulfills further important functions, inparticular it has to have a high mechanical stability and serve asseparator for the two fuels mentioned at the outset.

Significant advantages of such a membrane doped with phosphoric acid isthe fact that a fuel cell in which such a polymer electrolyte membraneis used can be operated at temperatures above 100° C. without themoistening of the fuel cell which is otherwise necessary. This is due tothe ability of the phosphoric acid to transport protons withoutadditional water by means of the Grotthus mechanism (K. -D. Kreuer,Chem. Mater. 1996, 8, 610-641).

The possibility of operation at temperatures above 100° C. results infurther advantages for the fuel cell system. Firstly, the sensitivity ofthe Pt catalyst to impurities in the gas, in particular CO, is greatlyreduced. CO is formed as by-product in the reforming of thehydrogen-rich gas comprising carbon-containing compounds, e.g. naturalgas, methanol or petroleum spirit, or as intermediate in the directoxidation of methanol. The CO content of the fuel typically has to beless than 100 ppm at temperatures of <100° C. However, at temperaturesin the range 150-200° C., 10 000 ppm or more of CO can also be tolerated(N.J. Bjerrum et al. Journal of Applied Electrochemistry, 2001, 31,773-779). This leads to significant simplifications of the upstreamreforming process and thus to cost reductions for the total fuel cellsystem.

The performance of a membrane-electrode unit produced using suchmembranes is described in WO 01/18894 A2. In a 5 cm² cell, at a gas flowof 160 ml/min and a gauge pressure of 1 atm for pure hydrogen and at agas flow of 200 m/min and a gauge pressure of 1 atm for pure oxygen.However, the use of pure oxygen, such a high gauge pressure and suchhigh stoichiometries is of no interest in industry.

The performance of such polyazole membranes doped with phosphoric acidwhen using pure hydrogen and pure oxygen is likewise described inElectrochimica Acta, Volume 41, 1996, 193-197. At a platinum loading of0.5 mg/cm² on the anode and 2 mg/cm² on the cathode, a current densityof less than 0.2 A/cm² at a voltage of 0.6 V is achieved when usinghumidified fuel gases consisting of pure hydrogen and pure oxygen at agauge pressure of 1 atm for each fuel gas. When air is used in place ofoxygen, this value drops to less than 0.1 A/cm².

A great advantage of fuel cells is the fact that the electrochemicalreaction converts the energy of the fuel directly into electric energyand heat. Water is formed as reaction product at the cathode. Heat isthus generated as by-product in the electrochemical reaction. In thecase of applications in which only the electric power is utilized fordriving electric motors, e.g. in automobile applications, or asreplacement for battery systems in many applications, part of the heatformed in the reaction has to be removed in order to avoid overheatingof the system. Additional, energy-consuming equipment is then necessaryfor cooling, and this further reduces the total electrical efficiency ofthe fuel cell. In the case of stationary applications such as central ordecentralized generation of power and heat, the heat can be utilizedefficiently by means of existing technologies, e.g. heat exchangers.High temperatures are sought here to increase the efficiency. If theoperating temperature is above 100° C. and the temperature differencebetween ambient temperature and the operating temperature is large, itis possible to cool the fuel cell system more efficiently or employsmall cooling areas and dispense with additional equipment compared tofuel cells which have to be operated at below 100° C. because of themoistening of the membrane.

However, besides these advantages, such a fuel cell system also hasdisadvantages. Thus, the durability of membranes doped with phosphoricacid is relatively limited. The life in this case is significantlyreduced by, in particular, operation of the fuel cell at below 100° C.,for example at 80° C. However, it needs to be stated in this contextthat the cell has to be operated at these temperatures during start-upand shutdown of the fuel cell.

Furthermore, the production of membranes doped with phosphoric acid isrelatively expensive, since it is usual firstly to form a polymer whichis subsequently cast with the aid of a solvent to produce a film. Afterdrying of the film, it is doped with an acid in a final step. Thepolymer membranes known hitherto therefore have a high content ofdimethylacetamide (DMAc) which cannot be removed completely by means ofknown drying methods.

In addition, the performance, for example the conductivity, of knownmembranes is still in need of improvement.

Furthermore, the durability of known high-temperature membranes having ahigh conductivity is still in need of improvement.

In addition, a very large amount of catalytically active substances isused to obtain a membrane-electrode unit.

It is therefore an object of the present invention to provide a novelpolymer electrolyte membrane which solves the abovementioned problems.In particular, a membrane according to the invention should be able tobe produced inexpensively and simply.

A further object of the present invention was to create polymerelectrolyte membranes which display good performance, in particular ahigh conductivity over a wide temperature range. This conductivityshould be able to be achieved without additional moistening, especiallyat high temperatures. The membrane should be able to be processedfurther to produce a membrane-electrode unit which can give particularlyhigh power densities. In addition, a membrane-electrode unit obtainableby use of the membrane according to the invention should have aparticularly good durability, in particular a long life at high powerdensities.

Furthermore, it was an object of the present invention to provide amembrane which can be converted into a membrane-electrode unit whichdisplays good performance even at a very low content of catalyticallyactive substances such as platinum, ruthenium or palladium.

A further object of the invention was to provide a membrane which can bepressed to form a membrane-electrode unit and allows the fuel cell to beoperated at low stoichiometries, at a low gas flow and/or at a low gaugepressure at a high power density.

Furthermore, the operating temperature should be able to be extended tothe range from <80° C. to 200° C. without the life of the fuel cellbeing greatly reduced.

These objects are achieved by a proton-conducting polymer membrane whichcomprises polyazoles and is coated with a catalyst layer and has all thefeatures of claim 1.

A membrane according to the invention displays a high conductivity overa wide temperature range, and this is also achieved without additionalmoistening. Furthermore, a membrane according to the invention can beproduced simply and inexpensively. In particular, large amounts ofexpensive solvents such as dimethylacetamide can be dispensed with.

Furthermore, these membranes display a surprisingly long life.Furthermore, a fuel cell equipped With a membrane according to theinvention can also be operated at low temperatures, for example at 80°C., without the life of the fuel cell being greatly reduced thereby.

In addition, the membrane can be processed further to produce amembrane-electrode unit which can give particularly high electriccurrents. A membrane-electrode unit obtained in this way has aparticularly good durability, in particular a long life at high electriccurrents.

Furthermore, the membrane of the present invention can be converted intoa membrane-electrode unit which displays good performance even at a verylow content of catalytically active substances such as platinum,ruthenium or palladium.

The present invention provides a proton-conducting polymer membranewhich comprises polyazoles and is coated with a catalyst layer and isobtainable by a process comprising the steps

-   A) preparation of a mixture comprising    -   polyphosphoric acid,    -   at least one polyazole (polymer A) and/or one or more compounds        which are suitable for forming polyazoles under the action of        heat according to step B),-   B) heating of the mixture obtainable according to step A) under    inert gas to temperatures of up to 400° C.,-   C) application of a layer using the mixture obtained according to    step A) and/or B) to a support,-   D) treatment of the membrane formed in step C) until it is    self-supporting,-   E) application of at least one catalyst layer to the membrane formed    in step C) and/or in step D).

The composition prepared in step B) comprises polyazoles. These polymerscan be added in step A) or they can be obtained from the monomers,oligomers and/or prepolymers on which the polymer is based during theheating in step B). Polymers based on polyazole comprise recurring azoleunits of the general formula (I) and/or (II) and/or (III) and/or (IV)and/or (V) and/or (VI) and/or (VII) and/or (VIII) and/or (IX) and/or (X)and/or (XI) and/or (XII) and/or (XIII) and/or (XIV) and/or (XV) and/or(XVI) and/or (XVII) and/or (XVIII) and/or (XIX) and/or (XX) and/or (XXI)and/or (XXII)

where

-   the radicals Ar are identical or different and are each a    tetravalent aromatic or heteroaromatic group which can be monocyclic    or polycyclic,-   the radicals Ar¹ are identical or different and are each a divalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar² are identical or different and are each a divalent    or trivalent aromatic or heteroaromatic group which can be    monocyclic or polycyclic,-   the radicals Ar³ are identical or different and are each a trivalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar⁴ are identical or different and are each a trivalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar⁵ are identical or different and are each a    tetravalent aromatic or heteroaromatic group which can be monocyclic    or polycyclic,-   the radicals Ar⁶ are identical or different and are each a divalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar⁷ are identical or different and are each a divalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar⁸ are identical or different and are each a trivalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals Ar⁹ are identical or different and are each a divalent    or trivalent or tetravalent aromatic or heteroaromatic group which    can be monocyclic or polycyclic,-   the radicals Ar¹⁰ are identical or different and are each a divalent    or trivalent aromatic or heteroaromatic group which can be    monocyclic or polycyclic,-   the radicals Ar¹¹ are identical or different and are each a divalent    aromatic or heteroaromatic group which can be monocyclic or    polycyclic,-   the radicals X are identical or different and are each oxygen,    sulfur or an amino group which bears a hydrogen atom, a group having    1-20 carbon atoms, preferably a branched or unbranched alkyl or    alkoxy group, or an aryl group as further radical,-   the radicals R are identical or different and are each hydrogen, an    alkyl group or an aromatic group and-   n, m are each an integer greater than or equal to 10, preferably    greater than or equal to 100.

Aromatic or heteroaromatic groups which are preferred according to theinvention are derived from benzene, naphthalene, biphenyl, diphenylether, diphenylmethane, diphenyldimethylmethane, bisphenone, diphenylsulfone, thiophene, furan, pyrrole, thiazole, oxazole, imidazole,isothiazole, isoxazole, pyrazole, 1,3,4-oxadiazole,2,5-diphenyl-1,3,4-oxadiazole, 1,3,4-thiadiazole, 1,3,4-triazole,2,5-diphenyl-1,3,4-triazole, 1,2,5-triphenyl-1,3,4-triazole,1,2,4-oxadiazole, 1,2,4-thiadiazole, 1,2,4-triazole, 1,2,3-triazole,1,2,3,4-tetrazole, benzo[b]thiophene, benzo[b]furan, indole,benzo[c]thiophene, benzo[c]furan, isoindole, benzoxazole, benzothiazole,benzimidazole, benzisoxazole, benzisothiazole, benzopyrazole,benzothiadiazole, benzotriazole, dibenzofuran, dibenzothiophene,carbazole, pyridine, bipyridine, pyrazine, pyrazole, pyrimidine,pyridazine, 1,3,5-triazine, 1,2,4-triazine, 1,2,4,5-triazine, tetrazine,quinoline, isoquinoline, quinoxaline, quinazoline, cinnoline,1,8-naphthyridine, 1,5-naphthyridine, 1,6-naphthyridine,1,7-naphthyridine, phthalazine, pyridopyrimidine, purine, pteridine orquinolizine, 4H-quinolizine, diphenyl ether, anthracene, benzopyrrole,benzooxathiadiazole, benzooxadiazole, benzopyridine, benzopyrazine,benzopyrazidine, benzopyrimidine, benzotriazine, indolizine,pyridopyridine, imidazopyrimidine, pyrazinopyrimidine, carbazole,aciridine, phenazine, benzoquinoline, phenoxazine, phenothiazine,acridizine, benzopteridine, phenanthroline and phenanthrene, which mayalso be substituted.

Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸, Ar⁹, Ar¹⁰, Ar¹¹ can have any substitutionpattern; in the case of phenylene, Ar¹, Ar⁴, Ar⁶, Ar⁷, Ar⁸, Ar⁹, Ar¹⁰,Ar¹¹ can be, for example, ortho-, meta- or para-phenylene. Particularlypreferred groups are derived from benzene and biphenylene, which mayalso be substituted.

Preferred alkyl groups are short-chain alkyl groups having from 1 to 4carbon atoms, e.g. methyl, ethyl, n- or i-propyl and t-butyl groups.

Preferred aromatic groups are phenyl and naphthyl groups. The alkylgroups and the aromatic groups may be substituted.

Preferred substituents are halogen atoms such as fluorine, amino groups,hydroxy groups or short-chain alkyl groups such as methyl or ethylgroups.

Preference is given to polyazoles having recurring units of the formula(I) in which the radicals X within one recurring unit are identical.

The polyazoles can in principle also have different recurring unitswhich differ, for example, in their radical X. However, preference isgiven to only identical radicals X being present in a recurring unit.

Further preferred polyazole polymers are polyimidazoles,polybenzthiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines,polythiadiazoles, poly(pyridines), poly(pyrimidines) andpoly(tetrazapyrenes).

In a further embodiment of the present invention, the polymer comprisingrecurring azole units is a copolymer or a blend comprising at least twounits of the formulae (I) to (XXII) which differ from one another. Thepolymers can be in the form of block copolymers (diblock, triblock),random copolymers, periodic copolymers and/or alternating polymers.

In a particularly preferred embodiment of the present invention, thepolymer comprising the recurring azole units is a polyazole comprisingonly units of the formula (I) and/or (II).

The number of recurring azole units in the polymer is preferably greaterthan or equal to 10. Particularly preferred polymers contain at least100 recurring azole units.

For the purposes of the present invention, polymers comprising recurringbenzimidazole units are preferred. Some examples of extremelyadvantageous polymers comprising recurring benzimidazole units arerepresented by the following formulae:

where n and m are each an integer greater than or equal to 10,preferably greater than or equal to 100.

The polyazoles used in step A), but in particular thepolybenzimidazoles, have a high molecular weight. Measured as intrinsicviscosity, it is at least 0.2 dl/g, preferably from 0.3 to 10 dl/g andparticularly preferably from 1 to 5 dl/g.

Furthermore, the polyazoles can also be prepared by heating in step B).For this purpose, one or more compounds which are suitable for formingpolyazoles under the action of heat according to step B) can be added tothe mixture prepared in step A).

Mixtures comprising one or more aromatic and/or heteroaromatictetraamino compounds and one or more aromatic and/or heteroaromaticcarboxylic acids or derivatives thereof which have at least two acidgroups per carboxylic acid monomer are suitable for this purpose.Furthermore, one or more aromatic and/or heteroaromaticdiaminocarboxylic acids can be used for the preparation of polyazoles.

The aromatic and heteroaromatic tertraamino compounds include, interalia, 3,3′,4,4′-tetraaminobiphenyl, 2,3,5,6-tetraaminopyridine,1,2,4,5-tetraaminobenzene, bis(3,4-diaminophenyl) sulfone,bis(3,4-diaminophenyl) ether, 3,3′,4,4′-tetraaminobenzophenone,3,3′,4,4′-tetraaminodiphenylmethane and3,3′,4,4′-tetraaminodiphenyldimethylmethane and their salts, inparticular their monohydrochloride, dihydrochloride, trihydrochlorideand tetrahydrochloride derivates. Among these,3,3′,4,4′-tetraaminobiphenyl, 2,3,5,6-tetraaminopyridine and1,2,4,5-tetraaminobenzene are particularly preferred.

Furthermore, the mixture A) can comprise aromatic and/or heteroaromaticcarboxylic acids. These are dicarboxylic acids and tricarboxylic acidsand tetracarboxylic acids or their esters or their anhydrides or theiracid halides, in particular their acid halides and/or acid bromides. Thearomatic dicarboxylic acids are preferably isophthalic acid,terephthalic acid, phthalic acid, 5-hydroxyisophthalic acid,4-hydroxyisophthalic acid, 2-hydroxyterephthalic acid,5-aminoisophthalic acid, 5-N,N-dimethylaminoisophthalic acid,5-N,N-diethylaminoisophthalic acid, 2,5-dihydroxyterephthalic acid,2,6-dihydroxyisophthalic acid, 4,6-dihydroxyisophthalic acid,2,3-dihydroxyphthalic acid, 2,4-dihydroxyphthalic acid,3,4-dihydroxyphthalic acid, 3-fluorophthalic acid, 5-fluoroisophthalicacid, 2-fluoroterephthalic acid, tetrafluorophthalic acid,tetrafluoroisophthalic acid, tetrafluoroterephthalic acid,1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid,diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid,bis(4-carboxyphenyl) ether, benzophenone-4,4′-dicarboxylic acid,bis(4-carboxyphenyl) sulfone, biphenyl-4,4′-dicarboxylic acid,4-trifluoromethylphthalic acid,2,2-bis(4-carboxyphenyl)hexafluoropropane, 4,4′-stilbenedicarboxylicacid, 4-carboxycinnamic acid, or their C1-C20-alkyl esters orC5-C12-aryl esters, or their acid anhydrides or their acid chlorides.

The aromatic tricarboxylic acids or their C1-C20-alkyl esters orC5-C12-aryl esters or their acid anhydrides or their acid chlorides arepreferably 1,3,5-benzenetricarboxylic acid (trimesic acid),1,2,4-benzenetricarboxylic acid (trimellitic acid),(2-carboxyphenyl)iminodiacetic acid, 3,5,3′-biphenyltricarboxylic acid,3,5,4′-biphenyltricarboxylic acid.

The aromatic tetracarboxylic acids or their C1-C20-alkyl esters orC5-C12-aryl esters or their acid anhydrides or their acid chlorides arepreferably 3,5,3′,5′-biphenyltetracarboxylic acid,1,2,4,5-benzenetetracarboxylic acid, benzophenonetetracarboxylic acid,3,3′,4,4′-biphenyltetracarboxylic acid,2,2′,3,3′-biphenyltetracarboxylic acid,1,2,5,6-naphthalenetetracarboxylic acid,1,4,5,8-naphthalenetetracarboxylic acid.

The heteroaromatic carboxylic acids are heteroaromatic dicarboxylicacids and tricarboxylic acids and tetracarboxylic acids or their estersor their anhydrides. For the purposes of the present invention,heteroaromatic carboxylic acids are aromatic systems in which at leastone nitrogen, oxygen, sulfur or phosphorus atom is present in thearomatic. Preference is given to pyridine-2,5-dicarboxylic acid,pyridine-3,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid,pyridine-2,4-dicarboxylic acid, 4-phenyl-2,5-pyridinedicarboxylic acid,3,5-pyrazoledicarboxylic acid, 2,6-pyrimidinedicarboxylic acid,2,5-pyrazinedicarboxylic acid, 2,4,6-pyridinetricarboxylic acid,benzimidazole-5,6-dicarboxylic acid, and also their C1-C20-alkyl estersor C5-C12-aryl esters, or their acid anhydrides or their acid chlorides.

The content of tricarboxylic acids or tetracarboxylic acids (based ondicarboxylic acid used) is in the range from 0 to 30 mol %, preferablyfrom 0.1 to 20 mol %, in particular from 0.5 to 10 mol %.

Furthermore, the mixture A) can also comprise aromatic andheteroaromatic diaminocarboxylic acids. These include, inter alia,diaminobenzoic acid, 4-phenoxycarbonylphenyl 3′4′-diaminophenyl etherand their monohydrochloride and dihydrochloride derivatives.

Preference is given to using mixtures of at least two different aromaticcarboxylic acids in step A). Particular preference is given to usingmixtures comprising heteroaromatic carboxylic acids in addition toaromatic carboxylic acids. The mixing ratio of aromatic carboxylic acidsto heteroaromatic carboxylic acids is in the range from 1:99 to 99:1,preferably from 1:50 to 50:1.

These mixtures are, in particular, mixtures of N-heteroaromaticdicarboxylic acids and aromatic dicarboxylic acids. Non limitingexamples of dicarboxylic acids are isophthalic acid, terephthalic acid,phthalic acid, 2,5-dihydroxyterephthalic acid, 2,6-dihydroxyisophthalicacid, 4,6-dihydroxyisophthalic acid, 2,3-dihydroxyphthalic acid,2,4-dihydroxyphthalic acid, 3,4-dihydroxyphthalic acid,1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid,diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid,bis(4-carboxyphenyl) ether, benzophenone-4,4′-dicarboxylic acid,bis(4-carboxyphenyl) sulfone, biphenyl-4,4′-dicarboxylic acid,4-trifluoromethylphthalic acid, pyridine-2,5-dicarboxylic acid,pyridine-3,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid,pyridine-2,4-dicarboxylic acid, 4-phenyl-2,5-pyridinedicarboxylic acid,3,5-pyrazoledicarboxylic acid, 2,6-pyrimidinedicarboxylic acid,2,5-pyrazinedicarboxylic acid.

If a very high molecular weight is to be achieved, the molar ratio ofcarboxylic acid groups to amino groups in the reaction of tetraaminocompounds with one or more aromatic carboxylic acids or esters thereofwhich have at least two acid groups per carboxylic acid monomer ispreferably in the vicinity of 1:2.

The mixture prepared in step A) preferably comprises at least 0.5% byweight, in particular from 1 to 30% by weight and particularlypreferably from 2 to 15% by weight, of monomers for the preparation ofpolyazoles.

If the polyazoles are prepared directly from the monomers in thepolyphosphoric acid, the polyazoles have a high molecular weight. Thisis particularly true of the polybenzimidazoles. Measured as intrinsicviscosity, it is preferably in the range from 0.3 to 10 dl/g, inparticular in the range from 1 to 5 dl/g.

If tricarboxylic acids or tetracarboxylic acids are also present in themixture obtained in step A), they effect branching/crosslinking of thepolymer formed. This contributes to an improvement in the mechanicalproperties.

In a further variant of the present invention, the mixture prepared instep A) comprises compounds which are suitable for forming polyazolesunder the action of heat according to step B), with these compoundsbeing obtainable by reaction of one or more aromatic and/orheteroaromatic tetraamino compounds with one or more aromatic and/orheteroaromatic carboxylic acids or derivatives thereof which have atleast two acids group per carboxylic acid monomer or of one or morearomatic and/or heteroaromatic diaminocarboxylic acids in the melt attemperatures of up to 400° C., in particular up to 350° C., preferablyup to 280° C. The compounds to be used for preparing these prepolymershave been described above.

The polyphosphoric acid used in step A) is a commercial polyphosphoricacid as is obtainable, for example, from Riedel-de Haen. Thepolyphosphoric acids H_(n+2)P_(n)O_(3n+1) (n>1) usually have an assaycalculated as P₂O₅ (acidimetry) of at least 83%. It is also possible fora dispersion/suspension to be produced instead of a solution of themonomers.

The mixture produced in step A) and/or step B) can also comprisedissolved, dispersed or suspended polymer. Such polymers can also beadded to the mixture after step B).

Preferred polymers include, inter alia, polyolefins such aspoly(chloroprene), polyacetylene, polyphenylene, poly(p-xylylene),polyarylmethylene, polyarmethylene, polystyrene, polymethylstyrene,polyvinyl alcohol, polyvinyl acetate, polyvinyl ether, polyvinylamine,poly(N-vinylacetamide), polyvinylimidazole, polyvinylcarbazole,polyvinylpyrrolidone, polyvinylpyridine, polyvinyl chloride,polyvinylidene chloride, polytetrafluoroethylene,polyhexafluoropropylene, copolymers of PTFE with hexafluoropropylene,with perfluoropropyl vinyl ether, with trifluoronitroisomethane, withsulfonyl fluoride vinyl ether, with carbalkoxyperfluoroalkoxyvinylether, polychlorotrifluoroethylene, polyvinyl fluoride, polyvinylidenefluoride, polyacrolein, polyacrylamide, polyacrylonitrile,polycyanoacrylates, polymethacrylimide, cycloolefinic copolymers, inparticular ones derived from norbornene;

-   polymers having C—O bonds in the main chain, for example polyacetal,    polyoxymethylene, polyethers, polypropylene oxide,    polyepichlorohydrin, polytetrahydrofuran, polyphenylene oxide,    polyether ketone, polyesters, in particular polyhydroxyacetic acid,    polyethylene terephthalate, polybutylene terephthalate,    polyhydroxybenzoate, polyhydroxypropionic acid, polypivalolactone,    polycaprolactone, polymalonic acid, polycarbonate;-   polymers having C—S bonds in the main chain, for example polysulfide    ether, polyphenylene sulfide, polyether sulfone;-   polymers having C—N bonds in the main chain, for example polyimines,    polyisocyanides, polyetherimine, polyaniline, polyamides,    polyhydrazides, polyurethanes, polyimides, polyazoles, polyazines;-   liquid-crystalline polymers, in particular Vectra, and-   inorganic polymers, for example polysilanes, polycarbosilanes,    polysiloxanes, polysilicic acid, polysilicates, silicones,    polyphosphazenes and polythiazyl.

Furthermore, the mixture produced in step A) and/or step B) can alsocomprise polymers having covalently bound acid groups. These polymerscan also be added to the mixture after step B). These acid groupsencompass, in particular, sulfonic acid groups. The polymers modifiedwith sulfonic acid groups preferably have a content of sulfonic acidgroups in the range from 0.5 to 3 meq/g. This value is determined viathe ion exchange capacity (IEC).

To measure the IEC, the sulfonic acid groups are converted into the freeacid. For this purpose, the polymer is treated with acid in a mannerknown per se, and excess acid is removed by washing. The sulfonatedpolymer is for this purpose firstly treated in boiling water for 2hours. Excess water is subsequently dabbed off and the sample is driedat 160° C. in a vacuum drying oven at p<1 mbar for 15 hours. The dryweight of the membrane is then determined. The polymer which has beendried in this way is then dissolved in DMSO at 80° C. over a period of 1hour. The solution is subsequently titrated with 0.1 M NaOH. The ionexchange capacity (IEC) is then calculated from the consumption of acidto the equivalence point and the dry weight. Such polymers are known tothose skilled in the art. Thus, polymers containing sulfonic acid groupscan be prepared, for example, by sulfonation of polymers. Processes forthe sulfonation of polymers are described in F. Kucera et. al. PolymerEngineering and Science 1988, Vol. 38, No 5, 783-792. Here, thesulfonation conditions can be selected so that a low degree ofsulfonation is obtained (DE-A-19959289).

A further class of nonfluorinated polymers has thus been developed bysulfonation of high-temperature-stable thermoplastics. Thus, sulfonatedpolyether ketones (DE-A-4219077, WO96/01177), sulfonated polysulfones(J. Membr. Sci. 83 (1993) p. 211) or sulfonated polyphenylene sulfide(DE-A-19527435) are known.

U.S. Pat. No. 6,110,616 describes copolymers of butadiene and styreneand their subsequent sulfonation for use in fuel cells.

Furthermore, such polymers can also be obtained by polymerizationreactions of monomers comprising acid groups. Thus, perfluorinatedpolymers can be prepared as described in U.S. Pat. No. 5,422,411 bycopolymerization of trifluorostyrene and sulfonylmodifiedtrifluorostyrene.

These perfluorosulfonic acid polymers include, inter alia, Nafion® (U.S.Pat. No. 3,692,569). This polymer can, as described in U.S. Pat. No.4,453,991, be brought into solution and then be used as ionomer.

Preferred polymers having acid groups include, inter alia, sulfonatedpolyether ketones, sulfonated polysulfones, sulfonated polyphenylenesulfides, perfluorinated polymers containing sulfonic acid groups, asdescribed in U.S. Pat. No. 3,692,569, U.S. Pat. No. 5,422,411 and U.S.Pat. No. 6,110,616.

The mixture obtained in step A) is heated to a temperature of up to 400°C., in particular 350° C., preferably up to 280° C., in particular from100° C. to 250° C. and particularly preferably in the range from 200° C.to 250° C., in step B). This is carried out using an inert gas, forexample, nitrogen or a noble gas such as neon, argon. It has also beenfound that when aromatic dicarboxylic acids (or heteroaromaticdicarboxylic acids) such as isophthalic acid, terephthalic acid,2,5-dihydroxyterephthalic acid, 4,6-dihydroxyisophthalic acid,2,6-dihydroxyisophthalic acid, diphenic acid,1,8-dihydroxynaphthalene-3,6-dicarboxylic acid, bis(4-carboxyphenyl)ether, benzophenone-4,4′-dicarboxylic acid, bis(4-carboxyphenyl)sulfone, biphenyl-4,4′-dicarboxylic acid, 4-trifluoromethylphthalicacid, pyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylic acid,pyridine-2,6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid,4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazoledicarboxylic acid,2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic acid are used,the temperature in step B) is advantageously in the range up to 300° C.,preferably from 100° C. to 250° C.

In one variant of the process, the heating according to step B) can becarried out after formation of a sheet-like structure according to stepC).

In addition, the mixture prepared in step A) and/or B) can furthercomprise organic solvents. These can have a positive effect on theprocessability. Thus, for example, the rheology of the solution can beimproved so that it can be extruded or spread by means of a doctor blademore easily.

To achieve a further improvement in the use properties, fillers, inparticular proton-conducting fillers, and additional acids can also beadded to the membrane. The addition can be carried out, for example, instep A), step B) and/or step C). Furthermore, these additives can, ifthey are in liquid form, also be added after the polymerizationaccording to step D).

Nonlimiting examples of proton-conducting fillers are

-   sulfates such as CsHSO₄, Fe(SO₄)₂, (NH₄)₃H(SO₄)₂, LiHSO₄, NaHSO₄,    KHSO₄, RbSO₄, LiN₂H₅SO₄, NH₄HSO₄,-   phosphates such as Zr₃(PO₄)₄, Zr(HPO₄)₂, HZr₂(PO₄)₃, UO₂PO₄.3H₂O,    H₈UO₂PO₄, Ce(HPO₄)₂, Ti(HPO₄)₂, KH₂PO₄, NaH₂PO₄, LiH₂PO₄, NH₄H₂PO₄,    CsH₂PO₄, CaHPO₄, MgHPO₄, HSbP₂O₈, HSb₃P₂O ₁₄, H₅Sb₅P₂O₂₀,-   polyacids such as H₃PW₁₂O₄₀.nH₂O (n=21-29), H₃SiW₁₂O₄₀.nH₂O    (n=21-29), H_(x)WO₃, HSbWO₆, H₃PMo₁₂O₄₀, H₂Sb₄O₁₁, HTaWO₆, HNbO₃,    HTiNbO₅, HTiTaO₂, HSbTeO₆, H₅Ti₄O₉, HSbO₃, H₂MoO₄,-   selenites and arsenides such as (NH₄)₃H(SeO₄)₂, UO₂AsO₄,    (NH₄)₃H(SeO₄)₂, KH₂AsO₄, Cs₃H(SeO₄)₂, Rb₃H(SeO₄)₂,-   phosphides such as ZrP, TiP, HfP-   oxides such as Al₂O₃, Sb₂O₅, ThO₂, SnO₂, ZrO₂, MoO₃,-   silicates such as zeolites, zeolites (NH₄ ⁺), sheet silicates,    framework silicates, H-natrolites, H-mordenites, NH₄-analcines,    NH₄-sodalites, NH₄-gallates, H-montmorillonites,-   acids such as HClO₄, SbF₅,-   fillers such as carbides, in particular SiC, Si₃N₄, fibers, in    particular glass fibers, glass powders and/or polymer fibers,    preferably ones based on polyazoles.

These additives can be present in customary amounts in theproton-conducting polymer membrane, but the positive properties such ashigh conductivity, long life and high mechanical stability of themembrane should not be impaired too much by addition of excessiveamounts of additives. In general, the membrane after the treatmentaccording to step D) contains not more than 80% by weight, preferablynot more than 50% by weight and particularly preferably not more than20% by weight, of additives.

In addition, this membrane can further comprise perfluorinated sulfonicacid additives (0.1-20% by weight, preferably 0.2-15% by weight, veryparticularly preferably 0.2-10% by weight). These additives lead to anincrease in power, in the vicinity of the cathode to an increase in theoxygen solubility and oxygen diffusion and to a reduction in theadsorption of phosphoric acid and phosphate onto platinum. (Electrolyteadditives for phosphoric acid fuel cells. Gang, Xiao; Hjuler, H. A.;Olsen, C.; Berg, R. W.; Bjerrum, N.J. Chem. Dep. A, Tech. Univ. Denmark,Lyngby, Den. J. Electrochem. Soc. (1993), 140(4), 896-902, andPerfluorosulfonimides as an additive in phosphoric acid fuel cell.Razaq, M.; Razaq, A.; Yeager, E.; DesMarteau, Darryl D.; Singh, S. CaseCent. Electrochem. Sci., Case West. Reserve Univ., Cleveland, Ohio, USA.J. Electrochem. Soc. (1989), 136(2), 385-90.)

Nonlimiting examples of perfluorinated additives are:trifluoromethanesulfonic acid, potassium trifluoromethanesulfonate,sodium trifluoromethanesulfonate, lithium trifluoromethanesulfonate,ammonium trifluoromethanesulfonate, potassium perfluorohexanesulfonate,sodium perfluorohexanesulfonate, lithium perfluorohexanesulfonate,ammonium perfluorohexanesulfonate, perfluorohexanesulfonic acid,potassium nonafluorobutanesulfonate, sodium nonafluorobutanesulfonate,lithium nonafluorobutanesulfonate, ammonium nonafluorobutanesulfonate,cesium nonafluorobutanesulfonate, triethylammoniumperfluorohexanesulfonate, perfluorosulfonimides and Nafion.

The formation of the sheet-like structure in step C) is carried out bymeans of measures known per se from the prior art for polymer filmproduction (casting, spraying, spreading by doctor blade, extrusion).Suitable supports are all supports which are inert under the conditions.These supports include, in particular, films of polyethyleneterephthalate (PET), polytetrafluoroethylene (PTFE),polyhexafluoropropylene, copolymers of PTFE with hexafluoropropylene,polyimides, polyphenylene sulfides (PPS) and polypropylene (PP).

To adjust the viscosity, the solution can, if appropriate, be admixedwith a volatile organic solvent. In this way, the viscosity can be setto the desired value and the formation of the membrane can be madeeasier.

The thickness of the sheet-like structure formed in step C) ispreferably in the range from 10 to 4000 μm, more preferably from 15 to3500 μm, in particular from 20 to 3000 μm, particularly preferably from30 to 1500 μm and very particularly preferably from 50 to 1200 μm.

The treatment of the membrane in step D) is carried out, in particular,at temperatures in the range from 0° C. to 150° C., preferably attemperatures in the range from 10° C. to 120° C., in particular fromroom temperature (20° C.) to 90° C., in the presence of moisture orwater and/or water vapor. The treatment is preferably carried out underatmospheric pressure, but can also be carried out under superatmosphericpressure. It is important that the treatment is carried out in thepresence of sufficient moisture, as a result of which the polyphosphoricacid present is partially hydrolyzed to form low molecular weightpolyphosphoric acid and/or phosphoric acid and thus contributes tostrengthening of the membrane.

The partial hydrolysis of the polyphosphoric acid in step D) leads to astrengthening of the membrane and to a decrease in the layer thicknessand formation of a membrane. The strengthened membrane generally has athickness in the range from 15 to 3000 μm, preferably from 20 to 2000μm, in particular from 20 to 1500 μm, with the membrane beingself-supporting.

The strengthening of the membrane in step D) also increases itshardness, which can be determined by means of microhardness measurementin accordance with DIN 50539. For this purpose, the membrane isgradually loaded with a Vickers diamond up to a force of 3 mN over aperiod of 20 s and the penetration depth is determined. According tothis measurement, the hardness at room temperature is at least 5 mN/mm²and preferably 20 mN/mm², without this constituting a restriction. Atthese hardness values, the membranes are generally self-supporting. Theforce is subsequently kept constant at 3 mN for 5 s and the creep iscalculated from the penetration depth. In the case of preferredmembranes, the creep C_(HU) 0.003/20/5 under these conditions is lessthan 30%, preferably less than 15% and very particularly preferably lessthan 5%. The modulus determined by means of microhardness measurementYHU is at least 0.1 MPa, in particular at least 2 MPa and veryparticularly preferably at least 5 MPa, without this constituting arestriction. The hardness of the membrane relates both to a surface onwhich no catalyst layer is present and to a side bearing a catalystlayer.

The upper temperature limit for the treatment according to step D) isgenerally 150° C. If the action of moisture is extremely brief, forexample in the case of superheated steam, this steam can also be hotterthan 150° C. The duration of the treatment is critical for thetemperature upper limit.

The partial hydrolysis (step D) can also be carried out in chambershaving a controlled temperature and humidity, in which case thehydrolysis can be controlled in a targeted fashion in the presence of adefined amount of moisture. The humidity can be set to a specific valueby means of the temperature or saturation of the environment in contactwith the membrane, for example gases such as air, nitrogen, carbondioxide or other suitable gases or steam. The treatment time isdependent on the values selected from the above parameters.

The treatment time is also dependent on the thickness of the membrane.

In general, the treatment time ranges from a few seconds to someminutes, for example in the presence of superheated steam, or up toentire days, for example in air at room temperature and relatively lowatmospheric humidity. The treatment time is preferably from 10 secondsto 300 hours, in particular from 1 minute to 200 hours.

If the partial hydrolysis is carried out at room temperature (20° C.) bymeans of ambient air at a relative atmospheric humidity of 40-80%, thetreatment time is in the range from 1 to 200 hours.

The membrane obtained according to step D) can be self-supporting, i.e.it can be detached from the support without damage and subsequently, ifappropriate, be directly processed further.

The concentration of phosphoric acid and thus the conductivity of thepolymer membrane of the invention can be set via the degree ofhydrolysis, i.e. the time, temperature and ambient humidity. Accordingto the invention, the concentration of the phosphoric acid is reportedas mol of acid per mol of repeating units in the polymer. For thepurposes of the present invention, a concentration (mol of phosphoricacid per mol of repeating units of the formula (III), i.e.polybenzimidazole) of from 10 to 80, in particular from 12 to 60, ispreferred. Such high degrees of doping (concentrations) can be obtainedonly with difficulty, if at all, by doping of polyazoles withcommercially available ortho-phosphoric acid.

Various methods can be used for applying at least one catalyst layeraccording to step E). Thus, for example, a support provided with acatalyst-containing coating can be used in step C) in order to providethe layer formed in step C) with a catalyst layer.

Here, the membrane can be provided with a catalyst layer on one or bothsides. If the membrane is provided with a catalyst layer on only oneside, then the opposite side of the membrane has to be pressed onto anelectrode which does not have a catalyst layer. If both sides of themembrane are to be provided with a catalyst layer, the following methodscan also be employed in combination in order to achieve an optimumresult.

According to the invention, the catalyst layer can be applied by aprocess in which a catalyst suspension is used. Furthermore, it is alsopossible to use powders comprising the catalyst.

The catalyst suspension comprises a catalytically active substance. Suchsubstances include, inter alia, noble metals, in particular platinum,palladium, rhodium, iridium and/or ruthenium. These substances can alsobe used in the form of alloys with one another. Furthermore, thesubstances can also be used in alloys with base metals such as Cr, Zr,Ni, Co and/or Ti. The oxides of the abovementioned noble metals and/orbase metals can also be used.

In a particular embodiment of the present invention, the catalyticallyactive compounds are used in the form of particles which preferably havea size in the range from 1 to 1000 nm, in particular from 10 to 200 nmand more preferably from 20 to 100 nm.

The catalytically active particles which comprise the abovementionedsubstances can be used as metal powders, known as noble metal black, inparticular platinum and/or platinum alloys. Such particles generallyhave a size in the range from 5 nm to 200 nm, preferably in the rangefrom 10 nm to 100 nm.

Furthermore, the metals can also be used on a support material. Thissupport preferably comprises carbon which can be used, in particular, inthe form of carbon black, graphite or graphitized carbon black. Themetal content of these supported particles, based on the total weight ofthe particles, is generally in the range from 1 to 80% by weight,preferably from 5 to 60% by weight and particularly preferably from 10to 50% by weight, without this constituting a restriction. The particlesize of the support, in particular the size of the carbon particles, ispreferably in the range from 20 to 100 nm, in particular from 30 to 60nm. The size of the metal particles present thereon is preferably in therange from 1 to 20 nm, in particular from 1 to 10 nm and particularlypreferably from 2 to 6 nm.

The sizes of the various particles are means of the weight average andcan be determined by means of transmission electron microscopy.

The catalytically active particles described above are generallycommercially available.

Furthermore, the catalyst suspension can contain customary additives.These include, inter alia, fluoropolymers such aspolytetrafluoroethylene (PTFE), thickeners, in particular water-solublepolymers such as cellulose derivatives, polyvinyl alcohol, polyethyleneglycol, and surface-active substances.

Surface-active substances include, in particular, ionic surfactants, forexample fatty acid salts, in particular sodium laurate, potassiumoleate; and alkylsulfonic acids, alkylsulfonic acid salts, in particularsodium perfluorohexanesulfonate, lithium perfluorohexanesulfonate,ammonium perfluorohexanesulfonate, perfluorohexanesulfonic acid,potassium nonafluorobutanesulfonate, and also nonionic surfactants, inparticular ethoxylated fatty alcohols and polyethylene glycols.

Furthermore, the catalyst suspension can comprise constituents which areliquid at room temperature. These include, inter alia, organic solventswhich may be polar or nonpolar, phosphoric acid, polyphosphoric acidand/or water. The catalyst suspension preferably contains from 1 to 99%by weight, in particular from 10 to 80% by weight, of liquidconstituents.

Polar, organic solvents include, in particular, alcohols such asethanol, propanol and/or butanol.

Organic, nonpolar solvents include, inter alia, known thin film diluentssuch as thin film diluent 8470 from DuPont, which comprises turpentineoils.

Particularly preferred additives are fluoropolymers, in particulartetrafluoroethylene polymers. In a particular embodiment of the presentinvention, the weight ratio of fluoropolymer to catalyst materialcomprising at least one noble metal and, if appropriate, one or moresupport materials is greater than 0.1, preferably in the range from 0.2to 0.6.

The catalyst suspension can be applied to the membrane in step C) and/orstep D) by customary methods. Depending on the viscosity of thesuspension, which can also be in paste form, various methods by means ofwhich the suspension can be applied are known. Suitable methods includeprocesses for coating films, woven fabrics, textiles and/or papers, inparticular spray processes and printing processes such as templateprinting processes and screen printing processes, inkjet processes,roller application, in particular halftone roller application, slitnozzle application and doctor blade coating. The respective method andthe viscosity of the catalyst suspension are dependent on the hardnessof the membrane.

The viscosity can be influenced by the solids content, in particular theproportion of catalytically active particles, and the proportion ofadditives. The viscosity to be set is dependent on the method ofapplying the catalyst suspension, with the optimal values and theirdetermination being well known to those skilled in the art.

Depending on the hardness of the membrane, the bonding of catalyst andmembrane can be improved by heating and/or pressing. In addition, thebonding between membrane and catalyst is strengthened by a treatmentaccording to step b).

Furthermore, the application of a catalyst layer according to step E)can be carried out simultaneously with the treatment of the membraneuntil it is self-supporting according to step D). This can be effectedby, for example, a water-containing catalyst suspension being applied tothe sheet-like structure obtained in step C). For this purpose, thesuspension can be sprayed in the form of fine droplets onto thesheet-like structure formed in step C). Apart from water, the suspensioncan further comprise additional solvents and/or diluents. Depending onthe water content, curing of the membrane is effected in step D). Thewater content can accordingly vary within a wide range. The watercontent is preferably in the range from 0.1 to 99% by weight, inparticular from 1 to 95% by weight, based on the catalyst suspension.

In a particular embodiment of the present invention, the catalyst layeris applied by a powder process in step E). Here, a catalyst powder whichcan contain additional additives as described by way of example above isused.

The catalyst powder can be applied using, inter alia, spray processesand screen processes. In the case of the spray process, the powdermixture is sprayed onto the membrane by means of a nozzle, for example aslit nozzle. In general, the membrane provided with a catalyst layer issubsequently heated to improve the bonding between catalyst andmembrane. The heating can, for example, be achieved by means of a hotroller. Such methods and apparatuses for applying the powder aredescribed, inter alia, in DE 195 09 748, DE 195 09 749 and DE 197 57492.

In the screen process, the catalyst powder is applied to the membraneusing a shaking screen. An apparatus for applying a catalyst powder to amembrane is described in WO 00/26982. After application of the catalystpowder, the bonding between catalyst and membrane can be improved bymeans of heating and/or step D). Here, the membrane provided with atleast one catalyst layer can be heated to a temperature in the rangefrom 50 to 200° C., in particular from 100 to 180° C.

In addition, the catalyst layer can be applied in step E) by a method inwhich a catalyst-containing coating is applied to a support and thecatalyst-containing coating present on the support is subsequentlytransferred to the membrane obtained according to step C) and/or stepD). Such a method is described by way of example in WO 92/15121.

The support provided with a catalyst coating can, for example, beproduced by preparing an above-described catalyst suspension. Thiscatalyst suspension is subsequently applied to a support film, forexample a polytetrafluoroethylene film. After application of thesuspension, volatile constituents are removed.

The transfer of the coating comprising a catalyst can be carried out by,inter alia, hot pressing. For this purpose, the assembly comprising acatalyst layer and a membrane and a support film is heated to atemperature in the range from 50° C. to 200° C. and pressed under apressure of from 0.1 to 5 MPa. In general, a few seconds suffice to jointhe catalyst layer to the membrane. This time is preferably in the rangefrom 1 second to 5 minutes, in particular from 5 seconds to 1 minute. Ina particular embodiment of the present invention, the catalyst layer hasa thickness in the range from 1 to 1000 μm, in particular from 5 to 500μm, preferably from 10 to 300 μm. This value represents a mean which canbe determined by measuring the layer thickness in cross-sectionalmicrographs obtained by means of a scanning electron microscope (SEM).

In a particular embodiment of the present invention, the membraneprovided with at least one catalyst layer comprises from 0.1 to 10.0mg/cm², preferably from 0.3 to 6.0 mg/cm² and particularly preferablyfrom 0.3 to 3.0 mg/cm². These values can be determined by elementalanalysis on a sheet-like sample.

Subsequent to the treatment according to step D) and/or step E), themembrane can be additionally crosslinked on the surface by the action ofheat in the presence of oxygen. This hardening of the membrane achievesan additional improvement in the properties of the membrane. For thispurpose, the membrane can be heated to a temperature of at least 150°C., preferably at least 200° C. and particularly preferably at least250° C. The oxygen concentration in this process step is usually in therange from 5 to 50% by volume, preferably from 10 to 40% by volume,without this constituting a restriction.

Crosslinking can also be effected by action of IR or NIR (IR=infrared,i.e. light having a wavelength of more than 700 nm; NIR=near IR, i.e.light having a wavelength in the range from about 700 to 2000 nm or anenergy in the range from about 0.6 to 1.75 eV). A further method isirradiation with β-rays. The radiation dose here is in the range from 5to 200 kGy.

Depending on the desired degree of crosslinking, the duration of thecrosslinking reaction can vary within a wide range. In general, thisreaction time is in the range from 1 second to 10 hours, preferably from1 minute to 1 hour, without this constituting a restriction.

The polymer membrane of the invention displays improved materialsproperties compared to the previously known doped polymer membranes. Inparticular, it displays improved power compared to known doped polymermembranes. This is due, in particular, to an improved protonconductivity. At a temperature of 120° C., this is at least 0.1 S/cm,preferably at least 0.11 S/cm, in particular at least 0.12 S/cm. If themembranes of the invention comprise polymers having sulfonic acidgroups, the membranes also display a high conductivity at a temperatureof 70° C. The conductivity is dependent, inter alia, on the sulfonicacid group content of the membrane. The higher this proportion, thebetter the conductivity at low temperatures. In this case, a membraneaccording to the invention can be moistened at low temperatures. Forthis purpose it is possible, for example, to provide the compound usedas energy source, for example hydrogen, with a proportion of water.However, the water formed by the reaction is in many cases sufficient toachieve moistening.

The specific conductivity is measured by means of impedance spectroscopyin a 4-pole arrangement in the potentiostatic mode using platinumelectrodes (wire, 0.25 mm diameter). The distance between thecurrent-collecting electrodes is 2 cm. The spectrum obtained isevaluated using a simple model comprising a parallel arrangement of anohmic resistance and a capacitor. The specimen cross section of themembrane doped with phosphoric acid is measured immediately beforemounting of the specimen. To measure the temperature dependence, themeasurement cell is brought to the desired temperature in an oven andthe temperature is regulated by means of a Pt-100 resistance thermometerpositioned in the immediate vicinity of the specimen. After thetemperature has been reached, the specimen is maintained at thistemperature for 10 minutes before commencement of the measurement.

Possible fields of use of the polymer membranes of the inventioninclude, inter alia, use in fuel cells, in electrolysis, in capacitorsand in battery systems.

The present invention also provides a membrane-electrode unit comprisingat least one polymer membrane according to the invention. For furtherinformation on membrane-electrode units, reference may be made to thespecialist literature, in particular the patents U.S. Pat. No.4,191,618, U.S. Pat. No. 4,212,714 and U.S. Pat. No. 4,333,805. Thedisclosure of the abovementioned references [U.S. Pat. No. 4,191,618,U.S. Pat. No. 4,212,714 and U.S. Pat. No. 4,333,805] in respect of thestructure and the production of membrane-electrode units and also theelectrodes, gas diffusion layers and catalysts to be selected isincorporated by reference into the present description.

To produce a membrane-electrode unit, the membrane of the invention canbe joined to a gas diffusion layer. If the membrane is provided on bothsides with a catalyst layer, the gas diffusion layer does not have tohave a catalyst present on it before pressing. However, it is alsopossible to use gas diffusion layers provided with a catalyticallyactive layer. The gas diffusion layer generally displays electrodeconductivity. It is usual to employ sheet-like, electrically conductiveand acid-resistant structures for this purpose. These include, forexample, carbon fiber papers, graphitized carbon fiber papers, wovencarbon fiber fabrics, graphitized woven carbon fiber fabrics and/orsheet-like structures which have been made conductive by addition ofcarbon black.

A membrane-electrode unit according to the invention displays asurprisingly high power density. In a particular embodiment, preferredmembrane-electrode units give a current density of at least 0.1 A/cm²,preferably 0.2 A/cm², particularly preferably 0.4 A/cm². This currentdensity is measured in operation using pure hydrogen at the anode andair (about 20% by volume of oxygen, about 80% by volume of nitrogen) atthe cathode at atmospheric pressure (1013 mbar absolute, with open celloutlet) and a cell voltage of 0.6V. Particularly high temperatures inthe range 150-200° C., preferably 160-180° C., in particular 170° C. canbe used here.

The abovementioned power densities can also be achieved at a lowstoichiometry of the fuel gases on both sides. In a particularembodiment of the present invention, the stoichiometry is less than orequal to 2, preferably less than or equal to 1.5, very particularlypreferably less than or equal to 1.2

In a particular embodiment of the present invention, the catalyst layerhas a low noble metal content. The noble metal content of a preferredcatalyst layer present in a membrane according to the invention ispreferably not more than 2 mg/cm², in particular not more than 1 mg/cm²,very particularly preferably not more 0.5 mg/cm². In a particularembodiment of the present invention, one side of a membrane has a highermetal content than the opposite side of the membrane. The metal contenton the one side is preferably at least twice the metal content on theopposite side.

In one variant of the present invention, membrane formation can becarried out directly on the electrode rather than on a support. Thetreatment according to step D) can in this way be correspondinglyshortened, since the membrane no longer has to be self-supporting. Sucha membrane is also provided by the present invention.

The present invention further provides an electrode having aproton-conducting polymer coating which is obtainable by a processcomprising the steps

-   A) preparation of a mixture comprising    -   polyphosphoric acid,    -   at least one polyazole (polymer A) and/or one or more compounds        which are suitable for forming polyazoles under the action of        heat according to step B),-   B) heating of the mixture obtainable according to step A) under    inert gas to temperatures of up to 400° C.,-   C) application of a layer using the mixture obtained according to    step A) and/or B) to an electrode,-   D) treatment of the membrane formed in step C),-   E) application of a catalyst layer to the membrane formed in step C)    and/or in step D).

For the sake of completeness, it should be stated that all preferredembodiments of a self-supporting membrane apply analogously for amembrane applied directly to the electrode.

In a particular embodiment of the present invention, the coating has athickness of from 2 to 3000 μm, preferably from 2 to 2000 μm, inparticular from 3 to 1500 μm, particularly preferably from 5 to 500 μmand very particularly preferably from 10 to 200 μm, without thisconstituting a restriction.

The treatment according to step D) leads to a hardening of the coating.The treatment is carried out until the coating has a hardness which issufficient to enable it to be pressed to produce a membrane-electrodeunit. A sufficient hardness is ensured when a membrane treated in thisway is self-supporting. However, a lower hardness is sufficient in manycases. The hardness determined in accordance with DIN 50539(microhardness measurement) is generally at least 1 mN/mm², preferablyat least 5 mN/mm² and very particularly preferably at least 15 mN/mm²,without this constituting a restriction.

An electrode which has been coated in this way can be installed in amembrane-electrode unit which, if appropriate, has at least one polymermembrane according to the invention.

1. A proton-conducting polymer membrane which comprises polyazoles andis coated with a catalyst layer and is obtainable by a processcomprising the steps A) preparation of a mixture comprisingpolyphosphoric acid, at least one polyazole (polymer A) and/or one ormore compounds which are suitable for forming polyazoles under theaction of heat according to step B), B) heating of the mixtureobtainable according to step A) under inert gas to temperatures of up to400° C., C) application of a layer using the mixture according to stepA) and/or B) to a support, D) treatment of the membrane formed in stepC) until it is self-supporting, E) application of at least one catalystlayer to the membrane formed in step C) and/or in step D).
 2. Themembrane as claimed in claim 1, characterized in that the support usedin step C) has been provided with a catalyst-containing coating in orderto provide the layer formed in step C) with a catalyst layer.
 3. Themembrane as claimed in claim 1, characterized in that the steps D) andE) are carried out simultaneously, with the membrane obtained in step C)being treated until it is self-supporting and provided with a catalystlayer in one step.
 4. The membrane as claimed in claim 1, characterizedin that the catalyst layer is applied by means of a powder process as instep E).
 5. The membrane as claimed in claim 1, characterized in thatthe catalyst layer is applied in step E) by means of a process in whicha catalyst suspension is used.
 6. The membrane as claimed in claim 5,characterized in that the catalyst suspension comprises at least oneorganic, nonpolar solvent.
 7. The membrane as claimed in claim 5,characterized in that the catalyst suspension comprises phosphoric acidand/or polyphosphoric acid.
 8. The membrane as claimed in claim 1,characterized in that the catalyst layer is applied in step E) by meansof a process in which a catalyst-containing coating is applied to asupport and the catalyst-containing coating present on the support issubsequently transferred to the membrane obtained according to step C)and/or step D).
 9. The membrane as claimed in claim 8, characterized inthat the transfer of the catalyst-containing coating is effected by hotpressing.
 10. The membrane as claimed in claim 1, characterized in thatthe mixture prepared in step A) comprises compounds which are suitablefor forming polyazoles under the action of heat according to step B),with these compounds comprising one or more aromatic and/orheteroaromatic tetramino compounds and one or more aromatic and/orheteroaromatic carboxylic acids or derivatives thereof which have atleast two acid groups per carboxylic acid monomer, and/or one or morearomatic and/or heteroaromatic diaminocarboxylic acids.
 11. The membraneas claimed in claim 1, characterized in that the mixture prepared instep A) comprises compounds which are suitable for forming polyazolesunder the action of heat according to step B), with these compoundsbeing obtainable by reaction of one or more aromatic and/orheteroaromatic tertraamino compounds with one or more aromatic and/orheteroaromatic carboxylic acids or derivatives thereof which have atleast two acid groups per carboxylic acid monomer or of one or morearomatic and/or heteroaromatic diaminocarboxylic acids in the melt attemperatures of up to 400° C.
 12. The membrane as claimed in claim 10,characterized in that aromatic and/or heteroaromatic tetraaminocompounds used as compounds suitable for forming polyazoles comprisecompounds selected from the group consisting of3,3′,4,4′-tetraaminobiphenyl, 2,3,5,6-tetraaminopyridine and1,2,4,5-tetraaminobenzene.
 13. The membrane as claimed in claim 10,characterized in that aromatic and/or heteroaromatic carboxylic acids orderivatives thereof containing at least two acid groups per carboxylicacid monomer which are used as compounds suitable for forming polyazolescomprise compounds selected from the group consisting of isophthalicacid, terephthalic acid, phthalic acid, 5-hydroxyisophthalic acid,4-hydroxyisophthalic acid, 2-hydroxyterephthalic acid,5-aminoisophthalic acid, 5-N,N-dimethylaminoisophthalic acid,5-N,N-diethylaminoisophthalic acid, 2,5-dihydroxyterephthalic acid,2,5-dihydroxyisophthalic acid, 2,3-dihydroxyphthalic acid,2,3-dihydroxyphtalic acid, 2,4-dihydroxyphthalic acid,3,4-dihydroxyphthalic acid, 3-fluorophthalic acid, 5-fluoroisophthalicacid, 2-fluoroterephthalic acid, tetrafluorophthalic acid,tetrafluoroisophthalic acid, tetrafluoroterephthalic acid,1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid,diphenic acid, 1,8-dihydroxynaphthalene-3,6-dicarboxylic acid,bis(4-carboxyphenyl) ether, benzophenone-4,4′-dicarboxylic acid,bis(4-carboxyphenyl) sulfone, biphenyl-4,4′-dicarboxylic acid,4-trifluoromethylphthalic acid,2,2-bis(4-carboxyphenyl)hexafluoropropane, 4,4′-stilbenedicarboxylicacid, 4-carboxycinnamic acid, their C1-C20-alkyl esters, theirC5-C12-aryl esters, their acid anhydrides and their acid chlorides. 14.The membrane as claimed in claim 10, characterized in that the compoundssuitable for forming polyazoles comprise aromatic tricarboxylic acids,their C1-C20-alkyl esters or C5-C12-aryl esters or their acid anhydridesor their acid halides or tetracarboxylic acids, their C1-C20-alkylesters or C5-C12-aryl esters or their acid anhydrides or their acidhalides.
 15. The membrane as claimed in claim 14, characterized in thatthe aromatic tricarboxylic acids comprise compounds selected from thegroup consisting of 1,3,5-benzoltricarboxylic acid (trimesic acid);2,4,5-benzoltricarboxylic acid (trimellitic acid);(2-carboxyphenyl)iminodiacetic acid, 3,5,3′-biphenyltricarboxylic acid;3,5,4′-biphenyltricarboxylic acid, 2,4,6-pyridinetricarboxylic acid,benzene-1,2,4,5-tetracarboxylic acid;naphthalene-1,4,5,8-tetracarboxylic acid,3,5,3′,5′-biphenyltetracarboxylic acid, benzophenonetetracarboxylicacid, 3,3′,4,4′-biphenyltetracarboxylic acid,2,2′,3,3′-biphenyltetracarboxylic acid,1,2,5,6-naphthalenetetracarboxylic acid and1,4,5,8-naphthalenetetracarboxylic acid.
 16. The membrane as claimed inclaim 14, characterized in that the content of tricarboxylic acidsand/or tetracarboxylic acids is in the range from 0 to 30 mol % based ondicarboxylic acid used.
 17. The membrane as claimed in claim 10,characterized in that the compounds suitable for forming polyazolescomprise heteroaromatic dicarboxylic acids, tricarboxylic acids and/ortetracarboxylic acids containing at least one nitrogen, oxygen, sulfuror phosphorus atom in the aromatic.
 18. The membrane as claimed in claim17, characterized in that pyridine-2,5-dicarboxylic acid,pyridine-3,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid,pyridine-2,4-dicarboxylic acid, 4-phenyl-2,5-pyridinedicarboxylic acid,3,5-pyrazoledicarboxylic acid, 2,6-pyrimidinedicarboxylic acid,2,5-pyrazinedicarboxylic acid, 2,4,6-pyridinetricarboxylic acid,benzimidazole-5,6-dicarboxylic acid, and also their C1-C20-alkyl estersor C5-C12-aryl esters, or their acid anhydrides or their acid chloridesare used.
 19. The membrane as claimed in claim 10, characterized in thatthe compounds suitable for forming polyazoles comprise diaminobenzoicacid and/or its monohydrochloride and dihydrochloride derivatives. 20.The membrane as claimed in claim 1, characterized in that the heatingaccording to step B) is carried out after formation of a sheet-likestructure according to step C).
 21. The membrane as claimed in claim 1,characterized in that the treatment according to step D) is carried outat temperatures in the range from 0° C. to 150° C. in the presence ofmoisture.
 22. The membrane as claimed in claim 1, characterized in thatthe treatment of the membrane in step D) is in the range from 10 secondsto 300 hours.
 23. The membrane as claimed in claim 1, characterized inthat the membrane formed after step D) and/or step E) is crosslinked byaction of oxygen.
 24. The membrane as claimed in claim 1, characterizedin that a layer having a thickness of from 20 to 4000 μm is produced instep C).
 25. The membrane as claimed in claim 1, characterized in thatthe membrane formed after step D) has a thickness in the range from 15to 3000 μm.
 26. The membrane as claimed in claim 1, characterized inthat the catalyst layer has a thickness in the range from 0.1 to 50 μm.27. The membrane as claimed in claim 1, characterized in that thecatalyst layer comprises catalytically active particles which have asize in the range from 0.1 to 10 μm.
 28. The membrane as claimed inclaim 1, characterized in that the membrane provided with a catalystlayer comprises from 0.1 to 10 g/m² of a catalytically active substance.29. The membrane as claimed in claim 27, characterized in that thecatalytically active substance comprises particles comprising platinum,palladium, gold, rhodium, iridium and/or ruthenium.
 30. The membrane asclaimed in claim 28, characterized in that the catalytically activeparticles comprise carbon.
 31. A membrane-electrode unit comprising atleast one electrode and at least one membrane as claimed in claim
 1. 32.A fuel cell comprising one or more membrane-electrode units as claimedin claim 31.